1. Field of the Invention
The present invention relates generally to an electronic doorbell chime circuit and more particularly to an electronic doorbell chime circuit that is capable of operating with more than one pushbutton or doorbell switch and detect which pushbutton is being depressed based on the presence or absence of signal generated through a diode placed in parallel with one of the doorbell pushbuttons.
2. Description of the Related Art
Electronic doorbell circuits come in many varieties and are generally known in the art. Many prior art electronic doorbell circuits utilize a conventional power source, such as widely available 120 VAC power coupled to a transformer for stepping down the AC power to a lower voltage, for example 16, 20 or 24 volts. The secondary winding of the transformer is typically electrically coupled to at least one doorbell switch and a concomitant detector circuit for determining when the doorbell switch has been depressed, thereby initiating the broadcast of a ring tone. In systems employing multiple doorbell switches, it is known to enable the detector circuit to distinguish between a plurality of doorbells, thereby enabling the broadcast of different ring tones for different doorbell locations.
In some known doorbell pushbutton detection circuits, a diode is connected in parallel across one of the doorbell switches as a component of a power supply circuit for the detection circuitry. In some of these known systems, the polarity of the diode becomes critical to the proper functioning of the circuit, thereby requiring an installer to properly orient the diode in order to permit the detection circuit to operate. Often this is a particularly acute problem for systems purchased for installation by homeowners, since they lack the requisite knowledge of electricity and electronics necessary to properly orient the diode.
Other prior art systems have obviated this problem by providing detection circuitry that functions regardless of the orientation of the diode placed in parallel with one of the pushbuttons. For example, U.S. Pat. No. 5,894,262 to McCavit et al., incorporated herein by reference, teaches an electronic doorbell detection circuit that is insensitive to the polarity of the diode, and utilizes a comparator circuit to detect a threshold voltage indicative of pushbutton depression. However, systems such as these utilizing a plurality of doorbell pushbuttons require a plurality of threshold voltages to be detected so that the system is capable of determining which doorbell is being depressed. This, in turn, requires the use of a plurality of comparator circuits or the like to detect voltages at certain threshold levels to determine pushbutton operation.